1. Field of the Invention
The invention relates to an independent floating caisson with a relatively deep draft, known in the business as “deep caisson” and “spar”, especially for use as a support for platforms employed in connection with recovery of hydrocarbons from subsea formations.
2. Description of Related Art
There are a number of types of floating platform, such as for example drilling and production ships, tension leg platforms (TLPs), semi-submersible platforms (semi-submersibles) and so-called spar platforms.
An example of a tension leg platform (TLP) is disclosed in U.S. Pat. No. 4,685,833 (Iwamoto). The publication describes a system for use on an isolated well on the seabed, comprising a basis unit 11 on the seabed, a buoyancy body 13 and a prestressed riser arrangement 12. The riser arrangement anchors the buoyancy body by means of pre-tensioning to the basis unit.
Another example of a tension leg platform (TLP) is disclosed in NO 316267 (Børseth), describing a method and a device for providing a stabilising moment for a TLP platform which is attached and anchored to the seabed.
U.S. Pat. No. 3,408,821 (Redshaw) describes a floating, ballastable column.
A spar platform has a support which substantially comprises a relatively long, columnar structure floating in an approximately vertical position in the water, with one or more buoyancy chambers in an upper part and a stabilising weight in the spar support's lower part. An upper part of the spar support extends above the water line where it supports a platform with, e.g. a drill deck, processing plant or the like. The spar support's relatively slim, elongate shape and relatively deep draft permit this type of support to tackle heave motions better (i.e. longer heave natural periods) than other types of floating platform.
A spar support of this type is described in patent publication U.S. Pat. No. 4,702,321 (Horton). Further examples of spar type supports in different variants are described in patent publications WO 2005/113329 (Horton), U.S. Pat. No. 5,722,797 (Horton), U.S. Pat. No. 4,630,968 (Berthet et al.), U.S. Pat. No. 6,309,141 (Cox et al.), WO 98/29299 (Allen et al.), and U.S. Pat. No. 6,161,620 (Cox et al.).
A platform with a spar support is a well-established structure, employed substantially in maritime areas with relatively low wave periods, such as off Malaysia and in the Mexican Gulf. In these waters waves are encountered with a typical period (Tp) of 13-15 s for a 100-year state.
A design restriction for the spar support is excitation of heave motion at resonance, and the combination of heave and roll/pitch motions for waves with a long period. In maritime areas such as, e.g. the North Atlantic, the wave conditions are considerably more challenging than in the Mexican Gulf, and floating supports designed for the Norwegian Sea have to be designed for wave periods (Tp) of between 15 and 19 s in a 100-year state.
When the spar support's heave natural period is excited by waves, it will produce unacceptable motions. Another resonance lies in the coupling of heave and roll/pitch motions, often referred to as Mathieu instability or parametric excitation. This effect occurs at different sum frequencies of heave and roll/pitch.
Thus there is a need for a platform support of the spar type which is better suited for installation in maritime areas with long waves (i.e. high wave periods) than is the case with the known spar supports.